Activated carbon is the preferred material for use in preparing electrodes for carbon electrode capacitors. This activated carbon is prepared from a number of different sources such as coconut shells, wood, sugar, cellulosics and phenolic resins. After converting these materials to carbon under steam controlled conditions, the carbons are “activated” in a second step using steam or catalyzed with KOH, or carbon dioxide and KOH to increase the surface area to very high surface areas such as 1000 to 2400 m2/g. These activated carbons usually contain about 2% oxygen after they have been thoroughly dried and traces of inorganic salts. This oxygen is probably present as quinones, hydroquinones, phenols, carboxylic acids, furans and possibly ketones etc. with some nitrogen compounds—all of which under higher voltage conditions greater than 3 V. will undergo electrochemical oxidation/reduction as the voltage is increased past 3.3 V. At lower voltages, these functional groups actually improve the energy storage capacity of the carbon and are desirable at voltages below 3.2 V.
However as the voltage is pushed higher than 3.2 V a high leakage current with gas formation, an increase in the ESR (resistance), and loss of performance occurs.
Electrochemical double layer capacitors capable of high energy density, known as “super-capacitors”, have been assembled from a variety of materials. In general, it is desirable to construct super-capacitors with light weight materials and electrolytes that are stable and non reactive, as described in U.S. Pat. No. 5,260,855 issued to Kaschmitter et al, the teachings of which are hereby incorporated by reference. This type of super-capacitor incorporates electrodes based on carbon may be prepared from organic gels.
U.S. Pat. No. 6,902,683 to Smith et al, which is herein incorporated by reference relates to electrolytes of a complex salt formed by mixing of a tetraalkyl ammonium salt of hydrogen fluoride with an imidazolium compound in a nitrile solvent which operate at temperatures between −60 and 150° C.
The article of Ue in J. electrochem. Soc. Vol 141, No. 11, November 1994 entitled “Electrochemical Properties of Organic Liquid Electrolytes Based on Quaternary Onium Salts for Electrical Double-Layer Capacitors” which is herein incorporated by reference, discloses high permittivity solvents and onium salts for double-layer capacitors. Specifically studied were quaternary onium tetrafluoroborate salts which showed greater solubility in the solvents with good stability and conductivity.
U.S. Pat. No. 5,418,682 to Warren et al, which is herein incorporated by reference discloses a method of preparing tetraalkyl ammonium tetrafluoroborate salts for use as electrolytes with dinitrile mixtures as solvents.
U.S. Pat. Nos. 6,535,373 and 6,902,684 to Smith et al, which are herein incorporated by reference, disclose similar electrolytes which primarily utilize nitrile solvents.